This invention relates to a method and apparatus for the preparation of reinforced or filled synthetic resins by extrusion.
It is known in industry that the physical, chemical and other properties of extruded and injection molded synthetic resins may be enhanced by the incorporation in these resins of one or more of certain other materials, hereinafter called additives. One of these additive materials comprises filamentary reinforcing, such as glass fibers, another comprises a resin different from the primary resin, and others comprise materials for other purposes, such as fillers of low cost materials to provide a lower cost of product.
In the contents of these specifications and claims, the word "extruder" is understood to include both an extruder, having a rotating screw, used for the heat plastifying and extrusion of plastics materials and a reciprocating screw plasticizer, having a rotating and reciprocating screw, as used on injection molding machines and as is disclosed in U.S. Pat. No. 2,734,226. The word "resin" is understood to include any thermoplastic or thermosetting plastics materials that are capable of being extruded in the conventional single or multiple screw extruder or reciprocating screw plasticizer wherein the resin is introduced into the extruder or plasticizer through a feed opening, is melted and mixed by a rotating screw or screws and is discharged under pressure as a molten material through a discharge orifice. The word "additive" is understood to mean any material which is added to and mixed with the resin in the carrying out of the process of the present invention.
One method for incorporation of additives into the resin is to premix the resin and additive in a mixer or blender and then feed the mixture to a feed opening of a conventional extruder. Disadvantages of this method include: first, the requirement of a separate batch mixing operation with the associated equipment; second, the inability to change or adjust the proportion of additive to resin during the running of the extruder; third, the difficulty of feeding the extruder with the mixer, especially when the mixture consists of a high proportion of additive to resin; and, fourth, the fact that the additive must undergo the same conditions of temperature, pressure and mixing required to melt and mix the resin which in the case of many additives results in thermal or physical degradation of the additive.
An improvement to the above method is disclosed in U.S. Pat. No. 3,520,027 wherein both the resin and additive are separately fed to a common feed section of an extruder, or fed to a continuous mixer which then discharges to a common feed section of an extruder. This improvement alleviates somewhat the first and second disadvantages of the aforementioned method but the third and fourth disadvantages remain.
U.S. Pat. No. 3,304,282 discloses that the resin may be supplied in powder or granulate form through the feed inlet of an extruder and melted in the kneading zone. After the last melting and kneading operation in the kneading zone, glass fibers of short length are added and mixed with the melt. However, no method or apparatus is disclosed for introducing the fibers into the resin, and it is specifically pointed out in the patent that it is difficult technically to meter in these fibers uniformly.